Afroyim v. Rusk

In Afroyim v. Rusk, 387 U.S. 253 (1967), the U.S. Supreme Court overruled Perez v. Brownell, 356 U.S. 44 (1958), and held that the United States could not revoke the citizenship of a citizen even though a federal law authorized such revocation based on the citizen's voting in an election in a foreign country (Israel).

Justice Hugo Black wrote the opinion for the 5-4 Court, concluding as follows:

Citizenship is no light trifle to be jeopardized any moment Congress decides to do so under the name of one of its general or implied grants of power. In some instances, loss of citizenship can mean that a man is left without the protection of citizenship in any country in the world -- as a man without a country. Citizenship in this Nation is a part of a cooperative affair. Its citizenry is the country and the country is its citizenry. The very nature of our free government makes it completely incongruous to have a rule of law under which a group of citizens temporarily in office can deprive another group of citizens of their citizenship. We hold that the Fourteenth Amendment was designed to, and does, protect every citizen of this Nation against a congressional forcible destruction of his citizenship, whatever his creed, color, or race. Our holding does no more than to give to this citizen that which is his own, a constitutional right to remain a citizen in a free country unless he voluntarily relinquishes that citizenship.

The Court did note that someone may voluntarily relinquish his United States citizenship. Afroyim v. Rusk, 387 U.S. 253, 268 (1967).